Crevice solution chemistry

Recently, Brossia and Kelly [28] studied the influence of alloy sulfur on the crevice solution chemistry and nature of the surface in the crevice at initiation. Through the use of occluded solution analysis, high-resolution transmission electron mi-croscopy/energy dispersive spectroscopy (TEM/EDS), small-spot X-ray photoelectron spectroscopy (XPS), and electrochemical measurements, the dominance of the aqueous sulfide species on the occluded solution chemistry evolution the initiation of crevice corrosion was shown. MnS inclusions were shown to dissolve chemically to form sulfide. No other sulfur species... 

The description of the crevice solution chemistry is probably the most difficult step (at least from the corrosion or chemistry standpoint if not the computer programming) as a result of the lack of reliable data. The main problems are ... 

Stidhar, N. and Dunn, D. S., "Effect of Applied Potential on Changes in Solution Chemistry Inside Crevices on Type 304. Stainless Steel and Alloy 825, Paper 347, NACE CORROSION/94, NACE International, Baltimore, MD, 1994. 

However, the Fig. 29 shows that the effect of chloride content is low on the repassivation potential, particularly when looking at the lower bound of the scatter b and of the measured value which is almost constant for chloride contents in excess of 10 M. This is consistent with the presence of a nearly saturated solution and/or precipitated salt film in well developed crevice corrosion, and indeed such environments should be almost independent on the bulk solution chemistry. Thus, a repassivation potential which, according Pourbaix [36], is close to the local potential of an actively corroding crevice should be poorly or no dependent on the bulk environment and crevice geometry. 

Anodic dissolution of 304 SS in simulated crevice solutions. The numbers indicate the degree of saturation of the solution. (From Hakkarainen, T., Corrosion Chemistry in Pits, Crevices and Cracks, A. Turnbull, ed., Fler M esty s Stationery Office, London, 1987.)... 

Local corrosion sites are typified by (1) local chemistries that are commonly only loosely related to the bulk exposure environment, (2) the separation of anodic and cathodic sites, and (3) the localization of corrosion damage sites (i.e., within pits, crevices, and cracks). Since, within a local corrosion site, the reactive surface area to available solution volume can be very large, extreme environments (in terms of concentration, concentration gradients, pH) are often encountered. For the same geometric reasons, these environments are difficult to characterize. Extremely high corrosion current densities can be sustained within the local site by the presence of much lower cathodic current densities over a much larger available surface area outside the corrosion site. Finally, the existence of ionic and concentration gradients between the local corrosion site and the external environment introduces complex transport scenarios. 

These techniques have been used to study pitting reaction control mechanisms in stainless steel (29,30), iron (31), and nickel (32). The effect of bulk solution flow on pit dissolution rates for Fe, Ti, and A1 have also been made (33). Interrogation of artificial crevice chemistries by x-ray absorption techniques... 

Crevice corrosion occurs in some environments because the nature of the environment within the crevice becomes more aggressive over time. There is little movement of the corrodent within a crevice. Over time, small changes in chemistry because of minor localized corrosion may become magnified because the solution is not being replenished by the bulk solution. 

Williams, D.E., Westcott, C. and Fleischmann, M. (1987) The effect of solution variables on pit initiation as measured by statistical methods, in Corrosion Chemistry within Pits, Crevices and Cracks Proceedings of a Conference Held at the National Physical Laboratory, Teddington, Middlesex, October 1-3, 1984 (ed. A. Turnbull), HMSO, London, pp. 61-88. 

It is usual to consider that various classes of anodic mechanisms exist depending on the range of potential with respect to the passivity domain. Active dissolution taking place at potentials preceding the passivation on a film-free surface is of major importance for the homogeneous corrosion in weakly oxidizing media such as acidic solutions of stable anions (e.g. sulfuric, perchloric, phosphoric, hydrochloric). Localized corrosion in pits, crevices, cracks, etc. is also assirmed to proceed through active dissolution stabilized at passive potentials by ohmic drops and/or local chemistry. 

---